Hi Wink Wink,
Adding a USB should be easier than wiring a plug, as goates said let us know what PC / operating system you are starting with and we can be a bit more precise with a recommendation.
Using a router rather than a modem would not so much solve the problem as avoid it, as the internet conection would be vai an ethernet port into the PC so none of the bandwith on the USB would be used by your net connection, it also makes it easier to add wireless connectivity or provide more wired PC's with internet access. Mind you I was assuming you have an ethernet port on the laptop and PC whcih may not be a safe assumption!
Back to the USB card - are you in the UK? If so PM me.
Memetic.